Q: how world leaders use nationalism to pursue their goals both personal and natioal gain

A: One of the major causes of most of those conflicts has been nationalism of a different kind - one that gets out of hand, turns into hatred of others, and sparks violence, often of the most brutal form. [ That is especially true when leaders of states can convince people that they have somehow been treated abusively by the "other" or that members of "our" group who live outside "our" borders

need to be incorporated into the "homeland."
As far as interstate war is concerned, there is no more obvious example than World War II. Japan, Italy, and especially Germany were all led by leaders who stressed unmet nationalist goals and grievances in the years leading up to the outbreak of fighting in 1939. While psychologists and historians still debate exactly how this took place, there is little doubt that the intense emotions felt by leaders and followers alike contributed to the atrocities committed by people from all three of these countries.
Nationalism of only a slightly different sort has fueled much of the intrastate violence that has been the dominant form of intractable political conflict since the end of World War II. In some cases, the term nationalism itself may not be used at all in what are referred to as ethnic or other "sub-national" conflicts, as is the case with many of the conflicts taking place inside of multinational countries such as India. In other cases, there is no realistic possibility of creating ethnically pure states; there is, for instance, no way to envision Hutu or Tutsi states emerging out of either Rwanda or Burundi. The largest number of cases involve nationalities whose historical claims to state- or nation-ness are rather tenuous as in Kashmir, Chechnya, or most of the former Yugoslav republics. But, the people who take up arms in those conflicts share the same kind of deeply rooted emotions that gave rise to the Nazis in Germany and any other Volk or nation-based ideology. ]